This is the encoding that strings returned from 'format' are in,
so since we need all strings to be UTF-8 independently of the locale
encoding, we need to set this explicitly to UTF-8.
(Also guile docs say this is picked up from the locale encoding,
but even though we call setlocale(3), the default port encoding
isn't set, for whatever reason.)
https://bugzilla.gnome.org/show_bug.cgi?id=733881